As the number of wireless devices increases, there is an endeavour to increase resource utilization in radio frequency spectrum.
Licensed radio frequency spectrum, for which long term evolution (LTE) is designed, provides many benefits in terms of network planning and quality-of-service guarantees, in relation to an unlicensed radio frequency spectrum. Since the amount of licensed spectrum is limited and has a price in terms of license cost, many operators exploit unlicensed spectrum, which comes at no licensing cost, as a complement in order to offload the LTE networks. In most cases, WiFi based on the IEEE 802.11 family of technologies is the technology used. Although WiFi provides means to access unlicensed spectrum, it has several drawbacks such as limited support for mobility and quality-of-service handling. Recently, the interest in using LTE for accessing unlicensed spectrum has increased.
Carrier aggregation, where a wireless device receives or transmits on multiple component carriers, is an integral part of LTE from release 10 onwards. In the LTE specifications, the component carriers correspond to a primary cell (PCell) and secondary cells (SCells).
From the perspective of the wireless device, there is only one PCell, whereas there may be one or more SCells. Cross-carrier scheduling is also supported, in which case downlink assignments and uplink scheduling grants relating to one carrier, e.g. an SCell, may be sent on another carrier, e.g. the PCell, using the (enhanced) physical downlink control channel ((E)PDCCH). Similarly, uplink control signalling on physical uplink control channel (PUCCH) from a user equipment (UE) to an evolved NodeB (eNodeB) is transmitted on the PCell regardless of whether it relates to the PCell or a SCell.
One possibility for accessing unlicensed spectrum with LTE is to build on the carrier aggregation framework already part of LTE, where the primary carrier corresponding to a PCell operates in a licensed spectrum whereas one or more secondary carriers corresponding to one or more SCells operate in an unlicensed spectrum.
The PCell is used for all mobility procedures, handles all critical control signalling, as well as user data, whereas the one or more SCells are used for best effort user data. This approach allows exploiting the unlicensed spectrum for LTE users without scarifying mobility and quality-of-service support. In addition, the operator only needs to handle one network.
An alternative to carrier aggregation is dual connectivity framework currently being developed in 3GPP, for multiple component carriers. In dual connectivity the carriers are associated with different base stations. Dual connectivity applied to licensed and unlicensed spectra, provides flexibility as the licensed and unlicensed accesses are implemented in separate nodes.
This is in contrast to carrier aggregation, where a PCell and a SCell are co-located in the same network node or base station.
Before an LTE wireless device may communicate with an LTE radio network node, the wireless device has to find and acquire synchronization to a cell within the LTE network and determine the identity of the cell found. This process is known as cell search. To assist the wireless device in this process, LTE defines two signals, the primary and secondary synchronization signals (PSS and SSS), which are transmitted from every LTE cell. The PSS/SSS are transmitted regularly every 5 ms. By measuring on these signals, the wireless device may establish time and frequency synchronization with the cell. Furthermore, different cells use different sequences and the wireless device may therefore establish the physical-layer cell identity by observing which PSS and SSS sequence in the set of possible sequences the cell in question used. Once synchronization to a cell is obtained, the wireless device may receive system information transmitted by each cell to obtain information necessary for accessing the system. The system information contains the so-called public land mobile network identity (PLMN ID), which is a globally unique identity of the operator to which the cell belongs. The PSS/SSS pair to use in a specific cell is determined by the operator as part of the network planning Since LTE operates in licensed spectrum, the same set of PSS/SSS sequences may be used by multiple operators as they are assigned different carrier frequencies.
For operation in unlicensed spectrum, as well as part of general enhancements to LTE in other areas such as coordinated multipoint transmission and reception (CoMP), so-called discovery signals are discussed.
A discovery signal is a sequence or set of sequences, typically of orthogonal frequency division multiplexing (OFDM) symbols, which are transmitted infrequently, e.g. a few times per second, from a transmission point, or a radio network node. A discovery signal may comprise un-modulated tones transmitted on a sequence of OFDM symbols.
By searching for discovery signals, a wireless device may find the transmission point and report e.g. the received signal quality to the network, which may use this information to determine whether the transmission point should be used for transmission to that wireless device or not.
In case of operation in unlicensed spectrum, each radio network node that transmits in unlicensed spectrum also transmits a discovery signal. Based on wireless device measurements on observed discovery signals, the radio network node may determine whether the wireless device should receive transmissions from a SCell that is operating in unlicensed spectrum.
The radio network node may configure the wireless device to search for a particular set of discovery signals. Alternatively, the wireless device searches over the full set of discovery signals without receiving information from the radio network node about the subset of discovery signals to search for. Upon detection of a discovery signal, the wireless device may report the signal quality back to the cell to which it is connected, after which the radio network node may, based upon this, take the desired action.
Transmissions in LTE are fully scheduled, i.e. a radio network node, such as an eNodeB is in control of when and on what resources wireless device, such as a UE shall be transmitting.
FIG. 1 schematically presents a network 16, in which a UE, 12 is served by an eNodeB 14.
In contrast to LTE, transmissions in WiFi are not scheduled but are autonomously handled.
FIG. 2 presents a scheme for transmission in WiFi, illustrating a first node 20, and a second node, 22 attempting unscheduled transmission. When the first node 20 has data to transmit, it listens to the channel activity for a certain amount of time, for example 20 microseconds (μs), and assesses whether the channel is available for transmission. Since the second node 22 is not transmitting any data during the listening time of the first node 20, the first node, 20 assesses that the channel is available for transmission, and may thus start transmission on the channel.
If the second node 22, assesses the channel availability during transmission by the first node 20, the second node 22 assesses that the channel as not available for uplink transmission. The second node 22 then waits a “back-off duration” in time, after which it assesses the channel availability again. Since the first node 20 does not transmit any data during this time, the second node 22 declares the channel available for transmission, after which it may transmit uplink data.
The scheme as presented in FIG. 2 is called listen before talk (LBT) since a node has to listen to the channel and assess the availability before it may transmit, i.e. talk. The use of LBT allows WiFi devices to share the spectrum among a multiple of other WiFi nodes. Moreover, the LBT allows WiFi devices to share the spectrum among non-WiFi devices.
There may also be regulatory requirements on LBT or similar schemes in some bands and regions.
When extending LTE to access an unlicensed spectrum on a SCell, it may be beneficial and may become a requirement to support LBT. In the downlink, the eNodeB may listen on the channel prior to the start of a subframe and, if the channel is declared available, schedule data transmissions in the subframes following the listening period.
The same principle may be applied in the uplink. If the UE finds the channel available, it follows the scheduling grant from the eNodeB and transmits in the uplink, otherwise it ignores the grant.
Preferably, the LBT period for all UEs connected to the same unlicensed node overlap as transmissions within that SCell are coordinated through scheduling. Uplink transmission should be avoided only when other nodes (e.g. WiFi) are currently using the channel.
Multiple operators may use the same unlicensed spectrum. Unless some other inter-operator coordination mechanism is used, the LBT period between different operators should in this case preferably not overlap as there is no scheduling coordination between the different operators. From one operator's perspective, another operator using LTE in unlicensed spectrum is no different from another operator using WiFi in unlicensed spectrum. A similar problem may also arise for different nodes belonging to the same operator if these nodes are not tightly coordinated.
In unlicensed spectrum, determining a discovery signal sequence in unlicensed spectrum cannot rely on network planning since multiple operators may use the same standard and hence the same overall set of possible sequences in the same spectrum. This is in contrast to traditional PSS/SSS configuration, for cell planning, in licensed spectrum, where only one operator exists on each frequency in a given geographical area.
Also, linking the discovery signal sequence to use in a particular network node to the globally unique PLMN ID may not be a good idea either, as the set of possible sequences becomes very large due to the large number of possible PLMN IDs.
There is hence a need for a solution addressing these issues as discussed above.